Such sockets are known from German Patent No. 3,412,291 and have the advantage that the transverse and longitudinal recess members and the corner pieces can easily be assembled and dismantled as a set. It has now been found that, especially in large heavy control panels, the socket does not have the required stability in the area of the corner pieces. The two vertical outside walls of the corner piece do not retain their right-angle relationship, and the panel-mounting flanges also slip out of horizontal alignment under load. This results in deformation of the socket so that a definite mounting plane is no longer provided for the control panel nor is there even a definite installation plane.